There may well be more mundane explanations for the surprising finding—explanations that'd have to be ruled out before gorillas could be reclassified as meat-eaters, said study co-author Grit Schubert, a primatologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

For example, gorillas are known to eat ants that scavenge the carcasses and bones of monkeys and other mammals. When gorillas eat the ants, they may also be ingesting—and later expelling—the mammal DNA in the ants' digestive tracts, the study authors speculate.

Another possibility is that the mammal DNA came from live monkeys or duikers that had been probing the gorilla feces for edible seeds or other leftover plant bits.

Or the mammals "might have just licked it, sniffed it, or peed on it," Schubert said.